Typically, it depends on how the ticket was opened and the SLA, but usually a ticket records both the phone number and the email addresses for the POC. An email is the preferred method of notification, because an out-of-office response typically lets you now right away if you are going to receive a response.
I assume that is 5 business days. If the 5 day rule is to continue, that should be within the first email. As well, as the POC information for the user to respond (both email and phone).

Does this apply to a technician engaging the user to investigate the incident or after the technician believes that the incident is resolved and is seeking confirmation from the POC? The latter (that it appears to be resolved), is normally 2 emails/48hrs and it is closed.

So if a customer is unresponsive you close the ticket within 5 business day.
Do you have a high rate of tickets re-opened?

In my company we currently follow this:
1st follow-up: email, after 2/3 days
2nd follow-up: email, 2 days after
3rd follow-up: phone call, 3 days after
4th follow-up: email, 2 days after and case closure.

So the total process take 2 weeks! This is much to long. But it's also true that customer in EMEA are usually slower to respond compare to our US based customers.
Obviously we have a very low rate of cases re-opened.

So if a customer is unresponsive you close the ticket within 5 business day.
Do you have a high rate of tickets re-opened?

In my company we currently follow this:
1st follow-up: email, after 2/3 days
2nd follow-up: email, 2 days after
3rd follow-up: phone call, 3 days after
4th follow-up: email, 2 days after and case closure.

So the total process take 2 weeks! This is much to long. But it's also true that customer in EMEA are usually slower to respond compare to our US based customers.
Obviously we have a very low rate of cases re-opened.

If you have a reasonable expectation that your POC has daily access to their email and you have provided them the ability to respond to the email (they can reply to actual person/team) and the option of calling by phone (you provide a direct number, not making them go through a queue), you should feel confident that if the POC required further assistance, they would have responded.
Do you utilize surveys? The email indicating the time has elapsed and the ticket is closed, should include a survey link or you should otherwise solicit a response. The complaints help you shape the amount of time you should allow.
Obviously, if you are assured they have email access and telephone access, you should close out the ticket within days, not weeks, especially if it is not a priority for them to respond. Although, if it involves something that they only utilize on a limited basis, the user may not be able to respond immediately.
Do you collect an alternate POC or a supervisor POC? Possibly adding the second POC, would allow for a greater possibility of response and a legitimate reason to close the ticket sooner, if you do not receive responses from either.
You may also want to collect metrics related to which individuals/groups are creating tickets and not responding. Those metrics can be utilized to make management aware of the situation to address the worst offenders.

72 hours (work days) seems reasonable, unless your userbase has some unique characteristic that makes them out of email and phone contact for prolong period of times. 3 days is more than enough time for someone to get around to hitting the 'reply' or returning a telephone call.

a) s/he is on leave/ill/on a course/no longer works in that area
you might as well have an arrangement to detect at least some of these reasons (e.g. try again in a week or so)

b) s/he does not understand or is unwilling
you probably cannot deal with this directly

Why not simply report all unanswered resolution confirmations to the customer manager who is the contact for that service. If you hold them frequently enough, you could even leave it to the next service review meeting.

This will not harm your resolution figures so long as:
a) you are not micro-managing them
b) you can distinguish 2 statuses (stati?) - "closed" and "work completed pending confirmation"
and c) you clear them with the customer reasonably frequently.

But it is all horses for courses and I can see no substitute for trying to resolve the question by agreeing a solution directly with the customer at the level at which you agree SLAs. come to think of it, it should be in your SLAs.

When IT services make up their own rules about such things, then, however well thought out they may be, they will be perceived as arbitrary (which they are) and they will be inferior to anything that can be achieved by the simple act of talking to the customer._________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718

To add...if it is an incident.. once the conditions for the cause of the incident has been resolve, the incident ticket should be resolve. the customer / point of contact is asked to verifiy if necessary ... depending on the service. Then after an agreed time, the ticket is close.

If the underlying service event happens after it has been resolved.. OPEN A NEW TICKET. ... rinse lather repeat

If it is a service request, then the time lines may be longer.. depending on how you classify the service request._________________John Hardesty
ITSM Manager's Certificate (Red Badge)